1. Field of the Invention
This invention relates to a reception apparatus for receiving e.g., digital broadcast by an orthogonal frequency division multiplexing (OFDM) system.
2. Description of Related Art
There has recently been proposed a modulation system termed an orthogonal frequency division multiplexing (OFDM) system, which is such a system in which a large number of orthogonal sub-carriers are provided in a transmission band and data are allocated to amplitude and phase of each sub-carrier to effect digital modulation in accordance with the phase shift keying (PSK) or the quadrature amplitude modulation (QAM).
This OFDM system has a feature that the total transmission speed is not changed from that in the conventional modulation system, even though the band per each sub-carrier is narrow and hence the modulation speed is lowered, because the transmission band is divided into a large number of sub-carriers. The system also has a feature that, since a large number of sub-carriers are transmitted in parallel, the symbol rate is lowered. So, with this OFDM system, the multi-pass time length with respect to the symbol time length can be shorter and hence is rendered less vulnerable to multi-path interference. Moreover, the OFDM system has a feature that, since data is allocated to plural sub-carriers, the transmission/reception circuitry can be formed by using inverse fast Fourier transform for modulation and fast Fourier transform (FFT) for demodulation to realize a transmission/reception circuit.
Referring to FIG. 1, transmission signals by the OFDM system is transmitted in terms of a symbol termed an OFDM symbol as a unit. This OFDM symbol is made up of an effective symbol, as an effective symbol during which IFFT takes place in transmission, and a guard interval which is direct copying of the waveform of a latter portion of the effective symbol. This guard interval is provided in a former portion of the OFDM symbol.
Because of the above characteristics, the OFDM system is extensively investigated as to the possibility of application thereof to terrestrial digital broadcast strongly influenced by the multi-pass interference. For the terrestrial digital broadcast, Digital Video Broadcasting-Terrestrial (DVB-T) or Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) has been proposed.
In each broadcast standard, which uses the OFDM system, such as DVB-T or ISDB-T, the length ratio of the guard interval to the effective symbol (guard interval ratio) can in general be selected from plural values depending on the difference in the contents of the information broadcast or on the characteristics of the transmission route.
For example, in the ISDB-T standard, it is recognized to use one of the values of ¼, ⅛, 1/16 and 1/32 as this guard interval ratio.
How this guard interval ratio is set can be changed e.g., from one channel to another, from one program to another or from one airing time to another, and can be optionally set by a broadcast furnishing side.
In each broadcast standard, which uses the OFDM system, carrier modulation processing of quadrature data modulation, data interleaving along the time axis for improving the anti-fading performance, and encoding by the punctured convolutional code, are used. In each broadcast system, one of plural carrier modulation scheme, one of plural time interleaving patterns and one of plural code rates can be selected, depending on the difference in the contents of the information aired or on the characteristics of the transmission route.
In e.g., the ISDB-T standard, one of DQPSK, QPSK, 16 QAM and 64 QAM can be adopted as the carrier modulation scheme. The time interleaving pattern can be selected from those having the delay amounts of 0 symbol, 2 symbols, 4 symbols, 8 symbols and 16 symbols, while the code rate of the convolutional code can be selected from among ½, ⅔, ¾, ⅚ and ⅞.
How the carrier modulation scheme, time interleaving pattern and the code rate of the convolutional code are set may be changed e.g., from one channel to another, from one program to another or from one airing time to another, and can be optionally set on the broadcast furnishing side.
Meanwhile, the OFDM reception apparatus performs synchronization control for determining the range of FFT operations termed a window synchronization control. Since this window synchronization control performs control of removing sample data of the guard interval length from the OFDM symbol, the reception apparatus needs to know the guard interval ratio of the OFDM signal being received. However, the guard interval ratio is optionally set on the broadcast furnishing side such that it cannot be unequivocally determined on the reception side.
Therefore, the conventional practice is to attempt achieving window synchronization, sequentially using the plural values of the guard interval ratio as set by the standard, and to find out a guard interval ratio that can be accurately demodulated as the demodulated signal is observed.
The result is the lengthened initial rising time until outputting of the sound and/or the picture as from the command for starting the reception until outputting of the sound and/or the picture.
In each broadcast standard, which uses the OFDM system, the manner of setting the carrier modulation scheme of the received signal, time interleaving patterns or the code rate of the convolutional code is stated in the transmission control information. For example, these are stated for the ISDB-T standard and for the DVB-T system in the transmission control information termed the transmission and multiplexing configuration control (TMCC) and in the transmission parameter signalling (TPS), respectively.
Consequently, the reception side is unable to perform decoding processing operations, such as time deinterleaving, demapping or viterbi decoding, until such time the transmission control information is demodulated reliably, with the result that the initial time as from a command for reception starting until sound and/or picture outputting is lengthened.